The major trend of the display market is shifting from the existing high-efficiency and high-resolution-oriented display to the emotional image-quality display aiming at realizing a high color purity for demonstration of natural colors. From this viewpoint, while organic light emitting diode (OLED) devices using organic light-emitters have remarkably developed, inorganic quantum dot LEDs with the improved color purity have been actively researched and developed as alternatives. However, in terms of materials, both the organic light-emitters and the inorganic quantum dot light-emitters have intrinsic limitations.
The existing organic light-emitters have an advantage of high efficiency, but the existing organic light-emitters have a wide spectrum and poor color purity. Although the inorganic colloidal quantum dot light-emitters have been known to have high color purity because the luminescence occurs by quantum confinement effects or the quantum size effects, the luminescent color varies depending on the sizes of nanoparticles, each of which has a diameter of below 10 nm. There is a problem that it is difficult to uniformly control the sizes of the quantum dots as the color approaches the blue color, and thereby the size distribution deteriorates the color purity because of size distribution. Furthermore, because the inorganic quantum dots have a very deep valence band, there is a problem that it is difficult to inject holes because a hole injection barrier from an organic hole injection layer or an anode is too large. Also, both the light-emitters (organic emitters and inorganic quantum dot emitters) are disadvantageously expensive. Thus, there is a need for new types of hybrid organic-inorganic light-emitters that compensate for the disadvantages of the organic light-emitters and inorganic quantum dot emitters and maintains their merits.
Since the emitting materials based on hybrid of organic and inorganic materials (hereafter, organic-inorganic-hybrid) have advantages of low manufacturing costs and simple manufacturing and device manufacturing processes and also have all advantages of organic emitting materials, which are easy to control optical and electrical properties, and inorganic emitting materials having high charge mobility and mechanical and thermal stability, the organic-inorganic-hybrid emitting materials are attracting attention academically and industrially.
Among them, since the organic-inorganic-hybrid perovskite materials have high color purity, simple color control, and low synthesis costs, the organic-inorganic-hybrid perovskite materials are very likely to be developed as the light-emitters. The high color purity (full width at half maximum (FWHM)≈20 nm) from these materials can be realized because they have a layered structure in which a two-dimensional (2D) plane made of the inorganic material is sandwiched between 2D planes made of the organic material and a large difference in dielectric constant between the inorganic material and the organic material is large (εorganic≈2.4, εinorganic≈6.1) so that the electron-hole pairs (or excitons) are bound to the inorganic 2D layer.
A material having the conventional perovskite structure (ABX3) is inorganic metal oxide.
In general, the inorganic metal oxides are oxides, for example, materials in which metal (alkali metals, alkali earth metals, lanthanides, etc) cations such as Ti, Sr, Ca, Cs, Ba, Y, Gd, La, Fe, and Mn, which have sizes different from each other, are located in A and B sites, oxygen anions are located in an X site, and the metal cations in the B site are bonded to the oxygen anions in the X site in the corner-sharing octahedron form with the 6-fold coordination. Examples of the inorganic metal oxides include SrFeO3, LaMnO3, CaFeO3, and the like.
On the other hand, since the organic-inorganic-hybrid perovskite has the ABX3 in which organic ammonium (RNH3) or inorganic cations are located in the A site, and halides (Cl, Br, I) are located in the X site to form the metal halide perovskite material, the organic-inorganic-hybrid perovskite are completely different from the inorganic metal oxide perovskite material in composition.
In addition, the materials vary in characteristics due to a difference in composition of the materials. The inorganic metal oxide perovskite typically has characteristics of superconductivity, ferroelectricity, colossal magnetoresistance, and the like, and thus has been generally conducted to be applied for sensors, fuel cells, memory devices, and the like. For example, yttrium barium copper oxides have superconducting or insulating properties according to oxygen contents.
On the other hand, since the organic-inorganic-hybrid perovskite (or inorganic metal halide perovskite) has a structure in which the organic plane (or “A site cation” plane in the perovskite crystal structure) and the inorganic plane are alternately stacked and thus has a structure similar to a lamellar structure so that the excitons are bound in the inorganic plane, it may be an ideal light-emitter that generally emits light having very high purity by the intrinsic crystal structure itself rather than the quantum size effect of the material.
If the organic-inorganic-hybrid perovskite has a chromophore (mainly including a conjugated structure) in which organic ammonium (or “A site cation” in perovskite crystals) has a bandgap less than that of an octahedron crystal structure composed of a central metal and a inorganic crystal structure (BX6), the luminescence occurs in the organic ammonium. Thus, since light having high color purity is not emitted, a full width at half maximum of the luminescence spectrum becomes wider than 50 nm. Therefore, the organic-inorganic-hybrid perovskite are unsuitable for a light emitting layer. Thus, in this case, it is not very suitable for the light-emitter having the high color purity, which is highlighted in this patent. Therefore, in order to produce the light-emitter having the high color purity, it is important that the luminescence occurs in an inorganic lattice composed of the central metal-halogen elements without the organic ammonium which does not contain the chromophore. That is, this patent focuses on the development of the light-emitter having high color purity and high efficiency in the inorganic lattice.
For example, although an electroluminescent device in which an organic-inorganic-hybrid material containing an emitting dye is formed in the form of a thin film rather than that of a particle and used as a light emitting layer, the emission originated from the emitting-dye itself, not from the intrinsic perovskite lattice crystal structure, as is disclosed in Korean Patent Publication No. 10-2001-0015084 (Feb. 26, 2001).
However, since the organic-inorganic-hybrid perovskite or the inorganic metal halide perovskite has small exciton binding energy, there is a fundamental problem that the luminescence occurs at a low temperature, but the excitons do not efficiently emit light at room temperature due to thermal ionization and delocalization of a charge carrier and thus they are easily separated as free charge carriers and then annihilated. Also, there is a problem in that the excitons are annihilated by the layer having high conductivity in the vicinity of the excitons when the free charge carriers are recombined again to form excitons.
Therefore, there is a need to study the perovskite materials having the improved luminescence efficiencies that are capable of being applied to various electronic devices.